The present invention relates to a centrifugal pressure filter with horizontal filter disks, the filtrate being discharged via a central hollow shaft. Centrifugal filters serve to remove solids from suspensions.
Centrifugal filters of the type described above are usually subjected to heavy loads, i.e. severe wear and tear, firstly due to the selected operating pressure and secondly by the suspension to be filtered, or more precisely, the eroding characteristics of the solid particles contained in the suspension. As a result, the filter must be checked at monthly to yearly intervals and it might be necessary to renew the screens on the filter disks just as often.
Conventional centrifugal pressure filters have a plurality of horizontal filter disks arranged on a central hollow shaft with a corresponding number of intermediate seals. The filtrate space of each filter disk is connected to the inside of the central hollow shaft via openings. The horizontal filter disk package and the central hollow shaft are arranged in a pressure casing and supported by bearings at both ends, i.e. at the top and bottom of the casing, in such a manner that the filter package can be rotated at high speed in order to centrifuge the filter cake. The suspension space between the casing and the filter package is isolated from the bearings of the central shaft by means of dynamic seals in order to prevent solid particles from penetrating into the bearings, as otherwise the bearings would be destroyed within a very short time.
Pressure filters which are known at present are operated in such a manner that the suspension containing solid particles is fed into the casing under pressure via intake nozzles when the pressure filter disk package is not rotating. The suspension is separated into filtrate and filter cake by means of the filter screen on the filter disks. The filtrate enters the filtrate space in the filter disks and the central hollow shaft and leaves the centrifugal filter through radial openings in the lower part of the central hollow shaft and through an outlet nozzle.
When the suspension is continuously being fed into the centrifugal filter, solids in the form of a filter cake settle on the filter disks after some time, i.e. after several hours or minutes, to such an extent that the throughput decreases considerably. The filter must then be cleaned. For this purpose, the suspension feed and the filtrate discharge are interrupted, the filter package on the central hollow shaft is rotated and the filter cake is then centrifuged from the filter disks. The filter cake is removed at the lowest point of the filter casing via a discharge opening.
If necessary, thorough cleaning of the filter disk screens can be achieved by means of backwashing. After the filter has been cleaned, the suspension feed is recommenced. In order to ensure continuous operation, suspension filtering units usually contain a number of centrifugal filters so that individual filters can be cleaned or serviced in turn without operation being interruped.
The servicing of a centrifugal filter takes up considerable time. When the filter casing is opened, the filter package, arranged on the central hollow shaft, is removed and placed outside the casing for inspection and, if necessary, the filter disks are relined. Otherwise the shaft is left in the casing and the filter disks are removed one by one, relined and then replaced on the central hollow shaft in the casing, alternately with the intermediate seals.
Centrifugal filters of the types known up to now, i.e. those with a removable shaft and those with a fixed shaft, have considerable disadvantages. The filter package arranged on the shaft can only be removed and inserted by means of a crane and the insertion of the shaft into the dynamic seals cannot be observed due to its inaccessibility. Consequently, these seals are likely to be damaged or not fitted properly resulting in poor sealing.
Placing the new filter disks and intermediate seals individually onto the shaft, which is arranged in the casing, takes up a considerable amount of time because of the lack of space available, and there is no possibility of observing the lowermost disks. In both cases, defects, such as short circuits for the flow of suspension, cannot be detected until after restart-up, i.e. until after the casing has been closed tight and all the feed and discharge lines have been connected. It has been found that, for known centrifugal filters, the leakage rate can be considerable after the filter disks have been relined and reassembled.